This invention relates to a variable frequency oscillator of a type suitable for simulating doppler frequency changes in a repeater or transponder and is particularly directed to means for shifting a frequency even though the frequency to be repeated may vary widely within a broad range.
In prior art sonar training devices and in sonar decoys, it has been found comparatively easy to receive, amplify and retransmit sonar signals to simulate an enemy submarine. Also, it has been easy to give a constant frequency shift to the received frequency to simulate relative motion between the observer and the decoy. But, if a constant doppler shift is given a frequency modulated signal, the targets will have an apparent velocity which is different for each frequency and hence would be unrealistic since true doppler is a fixed percentage of the frequency. With the advent of frequency modulated sonar signals and other variable frequency sonars where there is a rapid change in frequency, a system was required in which doppler frequency shift was a fixed percentage, or a slowly changing percentage, of the signal frequency.